Another long body of research focuses on a single simplification operation and can be broadly divided into three categories: (1) Lexical Simplification [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, where complex words are substituted with simpler words. (2) Syntactic Simplification [8]}, [9]}, [10]}, [11]}, which deals exclusively with sentence splitting, and (3) Sentence Compression [12]}, [13]}, [14]}, [15]}, [16]}, where the goal is to shorten the input sentence by removing its irrelevant content.
